I have been racking my brain trying to figure out what was wrong and couldn't see it until I read your post. Of course . . . the lenses is open too much. During a photography course that I took, it was pointed out that one should strive to use an f/stop somewhere in the middle of the range of the lens. I believe they referred to it as a "sweet spot". A rule of thumb perhaps, as each lens would most likely be different.

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"The best camera of tomorrow can't shoot a picture today".

Forgot to mention in the last quote. By the way about shooing wide open even though the lens is best one to two stops from wide open. I have no problem doing that and do so very frequently if I have to. It depends on the available light. I know I can get it snappy during PP. I'll take the faster shutter speeds, nice Bokeh and less noise over everything else.